The present invention relates to a method for stripping organic coatings from coated objects. More particularly, the present invention concerns a method for stripping protective coatings obtained from compositions based on organic resins and/or prepared with organic vehicles, such as paint, shellac, varnish, lacquer and the like, as well as residual coatings, including oils, asphalts, tars and synthetic polymers such as polystyrene. The method of the invention is especially useful for removing such coatings from objects having irregular surfaces and from large surfaces, including vertical and inclined surfaces in the interior of large constructions, such as storage bins and tanks on land and holds and ballast tanks of ships.
Commonly, paint is stripped from paint objects on a small scale by application of an organic or inorganic solvent or mixture thereof, as discussed in Kirk-Othmer's ENCYCLOPEDIA OF CHEMICAL TECHNOLOGY, Vol. 14, pp. 485-493, 2nd Edition, John Wiley and Sons, 1967.
Among solvents which many formulations employ, hydrocarbons, halocarbons, and particularly methylene chloride (dichloromethane) have been found to be particularly effective. Liquid stripping compositions usually contain additives including thickeners, evaporation retarders and detergents.
Organic solvent formulations for stripping paint and other coatings may be of the "scrape off" type or "flush off" type. Generally, the stripping composition is applied to the coated object by one of the foregoing methods and allowed to stand for some time, after which, the coating which has become swollen and/or softened is removed from the surface, by scraping, in the case of "scrape off" formulations or by flushing with water and/or by wiping with a damp rag in the case of "flush off" formulations.
The foregoing methods are expensive, since the organic solvent, except in the case of application by immersion, is not recovered. Moreover, all of the known procedures are generally dangerous and prohibitively expensive where large surfaces are involved. In addition, the waxes used as evaporation retarders in such formulations are difficult to remove completely and any residual wax interferes with the adhesion of subsequent coatings to the surfaces.
Processes have also been described in U.S. Pat. Nos. 2,689,198 to Judd; 3,794,524 to Nogueira et al and 3,832,235 to Cooper et al, wherein paint is stripped from a relatively small object by contact with the vapors from a boiling solvent composition. In these processes the hot vapors condense to liquids on the painted surface.
A method for cleaning tanks has been disclosed in U.S. Pat. No. 3,042,553 to Kearney et al wherein a solvent is heated to the boiling point to produce solvent vapors which are fed to a tank where they condense and wash the tank walls.
Such methods are not applicable for removing organic coatings from extensive surfaces because the cost of heating a sufficient amount of solvent to reflux for an extended period is prohibitive and moreover, expensive solvent-proof equipment is needed to carry out such an operation on a large scale. Furthermore, in some constructions, such as large metal tanks and ships, a temperature differential from one part of the construction to another can be harmful.
Systems wherein a liquid solvent mist is sprayed on a workpiece have also been used to clean surfaces, as disclosed for example in U.S. Pat. Nos. 4,101,340 to Rand and 2,756,168 to Ruggles. Spray washing requires a large amount of energy and therefore is undesirable in view of the expense and the increasing need to conserve energy.
It is the current industrial practice to remove paint and most other protective coatings from large tanks and other large constructions by the slow, unpleasant, expensive and dangerous procedure of abrasive blasting. It is important that a ballast tank of a ship, which usually carries ballast water, be rust-proof. To this end, ballast tanks are coated with a layer of paint. If the paint coating blisters or fails in any way, it is necessary to remove the paint from the interior of the ballast tank and repaint, to avoid the possibility of rust and eventual holes. This is especially important for ships which carry liquified natural gas. A ballast tank of a ship may have a capacity as large as one million gallons or more and often has a complex "honeycomb" configuration which makes it difficult and laborious for a blaster to work through. Removal and disposal of the large amount of blasting grit needed are costly.
To date, even though abrasive-blasting has severe disadvantages, it is practically the only procedure in use for removing paint from large surfaces; hydroblasting and even pounding with hammers are sometimes employed.
There is a tremendous demand for more effective and less labor-intensive methods for cleaning fixed storage tanks as well as tank cars of railroads and trucks, barge and ship holds of petroleum residues such as tar, pitch, asphalt, and oil, vegetable oil residues and residues of synthetic polymers such as polystyrene which result from storage of the monomer, preparatory to a change in type of cargo, structural repairs, or inspection by government agencies. Some of these tanks and holds have a capacity of more than 20,000,000 gallons.
At present they are cleaned mainly with hand-held high pressure streams of water or water-based solutions or emulsions, often followed by scraping with shovels and other hand tools. Caustic soda solutions may be used too. Costs are very high and the cleanliness achieved is often marginal or unacceptable. Residues are usually wasted and their disposal is a problem.
In our parent applications, Ser. Nos. 870,103 and 1,053, now U.S. Pat. Nos. 4,231,804 and 4,231,805, incorporated herein by reference, we have disclosed an economical process for stripping protective and residual coatings by means of a gaseous stripping composition wherein neither the gaseous stripping composition nor the surface being stripped is substantially above ambient temperature. While our process is surprisingly effective for removing almost all of such coatings, in some cases this process is slow for industrial use. Also, we have found that some coatings contain impurities or pigments which are insoluble in any practical stripping composition. For example, many oils and tarry substances, such as No. 6 Oil (Bunker C oil) and various tars and asphalts encountered in commerce contain small amounts of carbonized material and/or inorganic substances, such as rust particles, some of which are left on the surface, sometimes together with a small amount of the initial petroleum coating which is trapped in and around these particles after our process has been carried out. This residue generally amounts to less than 1% of the original coating and is usually unobjectionable; but sometimes, where a change of cargo is to take place, even such a minor amount of residue is undesirable.